Band 214, Heft 5 - 5

A new construction of a high temperature set-up for small angle X-ray scattering (SAXS) measurements on corrosive and volatile fluid systems is presented. The sample cell from single crystal sapphire is sealed with a special metal wire technique. The cell with the surrounding high temperature furnace is integrated in a homemade SAXS-spectrometer. This technique has been employed up to 1000°C for the example of SAXS-measurements on fluid K x KCl 1-x solutions which are briefly discussed.

Laser-pulse induced optoacoustic calorimetry (LIOAC) was applied to study dissipation as heat and temporary storage of light energy absorbed by TiO 2 particles (2.4 nm mean diameter) in aerated aqueous solution at pH 1-2.6 (HClO 4 ). The TiO 2 particles (1 g L -1 , 47 μM particle concentration) were excited by 6-ns laser pulses at 349 nm which produced ca. one e - /h + -pair per particle during each pulse. Light scattering was weak and was taken into account in the determination of absorbed photon energy (50-60 μJ per pulse). The LIOAC signal of the sample measured in a time window of 2 μs was analyzed with respect to that of a Na 2 CrO 4 solution used as a calorimetric reference. Deconvolution of the sample signal disclosed only one prompt heat dissipation process. From LIOAC at 8-18°C the fraction of prompt heat was (88±2)%, while (12±2)% of the absorbed molar photon energy was stored for at least 2 ms. The finding is attributed to fast formation of energy-storing species in competition with radiationless recombination of e - /h + -pairs. The energy balance is discussed in detail. Formation of trapped charge carriers (P1), oxidation of residual chloride (P2) and formation of quasi-free OH · (aq) and HO · 2(aq) radicals (P3) all persisting into the μs-regime are considered as energy-storing processes. Energy balance analysis shows that P3 is the main energy-storing reaction in the system studied here. The instantaneous quantum yield of OH · (aq) is estimated as (14±3)%. Strictu sensu it cannot be excluded that processes different from, however energetically similar to P3 effect temporary energy storage under the present conditions, since the nature of the species cannot be inferred from LIOAC. Implications of the findings are discussed.

Primary quantum yields Φ of NO 2 photolysis at wavelengths λ ≤ 398 nm and at bath gas pressures below 1 bar are analyzed. Stern-Volmer plots for collisional photolysis quenching, from experiments at pressures between 10 and 1000 bar, do not indicate a substantial reduction of the quantum yield below unity for pressures below 1 bar. The consequences of the recently discovered fluctuations of specific rate constants k(E) for NO 2 dissociation on collisional photolysis quenching are analyzed. These effects can lead to a small reduction of Φ at pressures below 1 bar which, however, is also smaller than the reduction reported in some experiments. Reanalysis of these experiments shows instead that, apart from experimental artifacts, the influence of the secondary reactions O + NO 2 (+ M) → NO 3 (+ M), NO 3 + NO → 2 NO 2 , and O + NO 2 → O 2 + NO was underestimated. As a consequence, all experimental evidence so far is in favour of a low pressure primary quantum yield which is unity over the complete wavelength range 300-398 nm. This leads to a revised recommendation of quantum yields Φ for the range 300-430 nm at 298 K and 248 K. A revision of the limiting low pressure rate constant at 298 K of the reaction O + NO 2 + N 2 → NO 3 + N 2 of (1.6±0.2)× 10 -31 [N 2 ] cm 3 molecule -1 s -1 is also recommended (to be employed together with F c = 0.6 and a limiting high pressure value of 2.2 × 10 -11 cm 3 molecule -1 s -1 ).

The thermal decomposition in solution of n -heptane of several aliphatic tert -butyl peroxyesters RC(O)OOTB: TB peroxypropionate, TB peroxy- n -butyrate, TB peroxy- iso -butyrate, TB peroxy-2-methylbutyrate, TB peroxy-2-ethylhexanoate, TB peroxy-3,5,5-trimethylhexanoate, and TB peroxy- neo -decanoate has been investigated at pressures up to 2500 bar and temperatures up to 185°C. The experiments were carried out in a tubular reactor at residence times up to 140 s. Peroxide concentration was monitored via quantitative FT-IR spectroscopic analysis of the pressurized medium under continuous flow conditions. For each peroxide first-order decomposition kinetics were observed over several half-lives. The type of carbon atom (in the R moiety) that is in α -position to the carbonyl group controls decomposition kinetics. The observed first-order rate coefficient, k obs , is smallest in situations where this particular C-atom is primary and is largest when it is tertiary. The ´´primary´´ TB peroxyesters are associated with the higher activation energies, around 145 kJ mol -1 . The activation energies, E A , of the ´´tertiary´´ and ´´secondary´´ TB peroxyesters are around 120 und 130 kJ mol -1 , respectively, with the individual E A ´s being proportional to the stability of the particular free-radical species R · . The activation volumes, Δ V ≠ obs , of the ´´secondary´´ and ´´tertiary´´ TB peroxyesters are in the narrow range 3.0±1.5 cm 3 mol -1 . Δ V ≠ obs of the ´´primary´´ TB peroxyesters is above 10 cm 3 mol -1 and, in addition, is slightly pressure-dependent. Both observations demonstrate that the ´´primary´´ aliphatic TB peroxyesters decompose via single-bond scission with this result being supported by modelling dissociation dynamics in the compressed fluid state. The ´´secondary´´ and ´´tertiary´´ TB peroxyesters decompose via concerted two-bond scission associated with the immediate formation of CO 2 . These results are fully consistent with evidence from the literature and from our preceding detailed study into TB peroxyacetate and TB peroxypivalate dissociation kinetics. The mode of primary bond dissociation, via single-bond or via concerted two-bond scission, largely affects the initiation efficiency of TB peroxyesters in free-radical polymerization. For each TB peroxyester, rate expressions for k obs ( p,T ) are presented. This data for decomposition in compressed n -heptane should be highly suitable for simulation and optimization of technical high-pressure polymerization processes.

The unimolecular dissociation of HOCl in the ground electronic state near the HO+Cl fragmentation threshold is investigated employing an ab initio potential energy surface. The dynamics calculations are performed by using an imaginary absorbing potential and filter diagonalization. The computational study is mainly focused on the influence of accidental or systematic vibrational resonances on the total angular momentum dependence of dissociation rates and the importance of Coriolis interaction.

Fluorinated hydrocarbon radical-radical reactions in the gas phase have been studied at low pressure (0.5 ≤ p/mbar ≤ 2) and low temperature (253 ≤ T/K ≤ 333) using the discharge flow reactor molecular beam sampling mass spectrometry (MS) technique. Stable and labile species have been detected by MS applying low energy electron impact as well as multiphoton ionisation. For the combination reaction CH 2 F + CH 2 F → products (1) the rate coefficient k 1 = (7.0±0.8) × 10 12 (T/298) (-3.9±1.0) cm 3 /mol × s was determined. At low pressure the HF elimination pathway (1b) CH 2 F + CH 2 F → C 2 H 3 F + HF (1b) is the main channel (k(1b)/k 1 = (0.82±0.015)). For the CHF 2 radical self reaction CHF 2 + CHF 2 → products (2) the rate coefficient was measured as k 2 = (1.7±0.5) × 10 13 exp((-555±89)/T) cm 3 /mol × s. The stabilisation of C 2 H 2 F 4 * is the main reaction pathway (2a) (k 2a /k 2 ≥ 0.5), the HF elimination is of minor importance (k 2b /k 2 = 0.2). No pressure dependence of k 1 and k 2 was observed in the limited pressure range.

The high-temperature phase of sodium ortho-phosphate, α -Na 3 PO 4 , is characterized by a dynamic rotational disorder of its polyatomic anions and, at the same time, by a considerable translational mobility of its cations. During the past decade, there has been considerable controversy about the question of whether both kinds of motion are dynamically coupled. To resolve this issue we have probed anionic and cationic motion individually over a wide range of experimental time scales. Coherent quasielastic neutron scattering as well as temperature-dependent 17 O NMR lineshape and relaxation spectroscopy serve to characterize the rotational motion of the anions, whereas the cation motion is probed by high-frequency conductivity and 23 Na NMR relaxation measurements. On the picosecond timescale, the combined interpretation of the neutron scattering and electrical conductivity data suggests strong dynamic coupling between the rotation of the phosphate groups about one of the four threefold P-O axes and the spatial fluctuations of nearby sodium ions. On more extended timescales, the NMR data indicate an additional, slower process, corresponding to dynamic jump reorientations of the C 3 axis of rotation. This process appears to be coupled to the translational Na + transport dynamics as suggested by a strong correspondence between the 17 O and 23 Na NMR relaxation characteristics and the electrical conductivities in the dc plateau region. The Na + transport process can be viewed as highly correlated, not unlike the chain mechanism observed in AgBr.

We outline the general theory as well as various approximations to the accurate calculation of vibrational and rotational transition frequency shifts between enantiomers of chiral molecules due to the parity violating weak nuclear interaction. The calculation of the effective parity violating potentials as a function of molecular geometry is mainly based on our recent, accurate Multiconfiguration-Linear Response approach (MC-LR, RPA and CASSCF, Berger and Quack, J. Chem. Phys. 112 (2000) 3148), which has been shown to lead to order of magnitude increases compared to early Restricted Hartree Fock (RHF) approaches. We present in some detail both the harmonic and diagonal anharmonic approach to parity violating vibrational frequency shifts. In these approaches the parity violating potentials are calculated as a function of all vibrational reduced normal coordinates q i with the relevant matrix elements beeing obtained from harmonic and anhar- monic vibrational wavefunctions. We also compare the adiabatic and reverse adiabatic harmonic approximation. In addition to the general exact theory several approximate approaches are introduced for the calculation of parity violating structural differences between enantiomers resulting in the corresponding changes of rotational transition frequencies. Results are presented for the chiral molecule CHBrClF. The predicted relative vibrational frequency shifts Δ pv ω i /ω i are shown to depend strongly on the vibrational mode and the level of calculation (RHF, RPA, CASSCF) but in all cases are on the order of 10 -(16±1) , much smaller than all previous experimental tests could detect. The predicted relative rotational frequency shifts fall in a similar range. We discuss consequences of our predictions for various possible experiments on molecular parity violation.